1. Field of the Invention
The present invention relates to a driving apparatus for a display panel which uses capacitive light emitting elements such as organic electroluminescence elements.
2. Description of the Related Background Art
An organic electroluminescence element (hereinafter simply called the EL element), which is one of capacitive light emitting elements, can be electrically represented by an equivalent circuit as shown in FIG. 1. As understood from FIG. 1, the EL element can be replaced by a configuration of a capacitive component C, and a component E having a diode characteristic coupled in parallel with the capacitive component. Therefore, the EL element can be regarded as a capacitive light emitting element. As a direct current light emission driving voltage is applied between electrodes, a charge is accumulated on the capacitive component C, and as it subsequently exceeds a barrier voltage or a light emission threshold voltage inherent to the element, a current begins flowing from an electrode (anode side of the diode component E) to an organic functional layer carrying a light emitting layer, causing the EL element to emit light at an intensity proportional to the current.
The voltage V—current I—luminance L characteristic of the EL element is similar to the characteristic of a diode, as shown in FIG. 2, where the current I is extremely small at a voltage equal to or lower than the light emission threshold voltage Vth, and the current I suddenly increases at a voltage equal to or higher than the light emission threshold voltage Vth. Also, the current I substantially proportional to the luminance L. Such an element presents a light emission luminance proportional to a current in accordance with a driving voltage when the element is applied with the driving voltage exceeding the light emission threshold voltage Vth, and no driving current flows if the applied driving voltage is equal to or lower than the light emission threshold voltage Vth, causing the light emission luminance to remain equal to zero.
A simple matrix display panel which has a plurality of EL elements arranged in matrix is known. FIG. 3 shows a simple matrix display panel 1 and a driving apparatus for driving the display panel 1. The display panel 1 shows a total of 16 EL elements E1,1–E4,4, four in the horizontal direction and four in the vertical direction, for simplifying explanation. Each of the EL elements E1,1–E4,4 forms a pixel of the display panel 1. In FIG. 3, each of the EL elements E1,1–E4,4 is represented by a symbol of a diode. The EL elements E1,1–E4,4 are arranged at portions at which four anode lines (drive lines) A1–A4 arranged in parallel in the vertical direction intersect with four cathode lines (scanning lines) B1–B4 arranged in parallel in the horizontal direction. An anode electrode of each of the EL elements E1,1–E4,4 is connected to a corresponding anode line A1–A4, while a cathode electrode is connected to a corresponding cathode line B1–B4.
The driving apparatus of the display panel 1 includes an anode driving circuit 2 and a cathode scanning circuit 3. The anode driving circuit 2 has current sources 41–44 and change-over switches 51–54 corresponding to the anode lines A1–A4. Input terminals of the current sources 41–44 are connected to a positive terminal of a power supply 7 for outputting a voltage VA (for example, 24 V). Current output terminals of the current sources 41–44 are each connected to one fixed contact of a change-over switch 51–54 corresponding thereto. The other fixed contacts of the change-over switches 51–54 are connected to a ground. Movable contacts of the change-over switches 51–54 are connected to corresponding anode lines A1–A4. Switching operations of the change-over switches 51–54 are controlled by a control circuit 9 in accordance with an image signal.
The cathode scanning circuit 3 has change-over switches 61–64. One fixed contact of the change-over switch 61–64 is connected to a positive terminal of the power supply for outputting a voltage VK (for example, 20 V), and the other fixed contacts are connected to the ground. Movable contacts of the change-over switches 61–64 are connected to the corresponding cathode lines B1–B4. Switching operations of the change-over switches 61–64 are sequentially performed by the control circuit 9. The control circuit 9 repeatedly supplies a selection signal to the change-over switches 61–64 in that order in synchronism with a horizontal synchronization signal of an image signal. The movable contacts of the change-over switches 61–64 are normally in contact with the one fixed contacts, and any one of the change-over switches 61–64 supplied with the selection signal from the control circuit 9 switches to a contact to the other fixed contact. A ground potential (0 V) is applied to the cathode lines B1–B4 in order through the change-over switch selected by the selection signal, thus performing the scanning.
A current flows through the change-over switches 51–54 into EL elements driven to emit light corresponding to an image signal within EL elements connected to a cathode line at the ground potential, i.e., a selected cathode line, so that the EL elements emit light.
FIG. 4 shows operating states of the change-over switches 51–54 and 61–64, and a current flow at the time the EL elements E1,2 and E3,2 begin emitting light. The change-over switches 51 and 53 connect the current sources 41 and 43 to the anode lines A1 and A3, and the change-over switches 52 and 54 apply the ground potential to the anode lines A2 and A4. The change-over switch 62 is selected to apply the ground potential to the cathode line B2, and the remaining change-over switches 61, 63 and 64 apply the potential VK to the cathode lines B1, B3 and B4.
In the state of FIG. 4, the current flows, as indicated by an arrow, from the current source 41 to the ground through the change-over switch 51, anode line A1, EL element E1,2, cathode line B2 and change-over switch 62, causing the EL element E1,2 to emit light. Similarly, the current flows from the current source 43 to the ground through the change-over switch 53, anode line A3, EL element E3,2, cathode line B2 and change-over switch 62, causing the EL element E3,2 to emit light.
Also, in the state of FIG. 4, the cathode electrodes of the EL elements E1,1–E4,1 are applied with the potential VK through the change-over switch 61 and cathode line B1; the cathode electrodes of the EL elements E1,3–E4,3 are applied with the potential VK through the change-over switch 63 and cathode line B3; and the cathode electrodes of the EL elements E1,4–E4,4 are applied with the potential VK through the change-over switch 64 and cathode line B4. The anode lines A2 and A4 are driven to the ground potential through the change-over switches 55 and 54. Thus, the EL elements E2,1, E2,3, E2,4, E4,1, E4,3 and E4,4 indicated by black diode symbols in FIG. 4 are applied with the voltage VK in the reverse direction in polarity, resulting in a so-called backward biased state. Currents flows through the EL elements E2,1, E2,3, E2,4, E4,1, E4,3 and E4,4 from the cathode electrode side to the anode electrode side, as indicated by arrows in FIG. 4, to charge them.
Upon termination of a light emitting period of the EL elements E1,2 and E3,2, the change-over switches 51 and 53 are switched, as shown in FIG. 5, to make the anode lines A1 and A3 equal to the ground potential. At this time, the change-over switch 62 still maintains the cathode line B2 to be equal to the ground potential. Therefore, since the anode electrodes and cathode electrodes of the EL elements E1,2, E2,2, E3,2 and E4,2 are both at the ground potential, the EL elements E1,2, E3,2 stop emitting light. On the other hand, the EL elements E1,1, E1,3, E1,4, E3,1, E3,3 and E3,4 indicated by black diode symbols in FIG. 5 are applied with the voltage VK in the reverse direction in polarity, resulting in a so-called backward biased state. Currents flow through the EL elements E2,1, E2,3, E2,4, E4,1, E4,3 and E4,4 from the cathode electrode side to the anode electrode side, as indicated by arrows in FIG. 5, to charge them. FIG. 6 shows an example of a change in the current flowing into an EL element when it is charged. In FIG. 6, charging is started from time t=0.
Charges accumulated on EL elements except for the EL elements E1,2, E2,2, E3,2 and E4,2 by the charging are discharged by a reset operation, immediately before the selection signal is generated for the next scanning, which forcedly connects all the cathode lines B1–B4 to the ground and applies a predetermined potential to the anode lines A1–A4.
Such charging and discharging operations are similar when any one of other change-over switches 61, 63, 64 is selected by scanning to supply the ground potential to the cathode lines.
However, in the conventional driving apparatus, there is a problem that a charging current flows through EL elements which are connected to cathode lines other than the cathode line selected by the scanning as described above and are not related to light emission, so that power is consumed uselessly.